Filter circuits based on trans-conductor circuits

ABSTRACT

A filter circuit uses a trans-conductor circuit coupled to a control circuit having similar characteristics as the trans-conductor circuit. The control circuit is used to set and/or control the quiescent voltage of the trans-conductor circuit. As a result, the trans-conductor circuit may be designed to operate across a large frequency range while consuming minimal power. The control circuit may be operated using direct current (dc) voltage and may be implemented similar to the trans-conductor circuit implemented in the filter circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to filter circuits, and more specificallyto a method and apparatus for implementing filter circuits based ontrans-conductor circuits.

2. Related Art

Filter circuits are generally implemented to perform correspondingtransfer functions as is well known in the relevant arts. By theappropriate choice of transfer functions, filter circuits may beimplemented to provide corresponding utilities (e.g, low pass filter,band pass filter). Filter circuits may be used in several environmentssuch as signal receiving systems, digital circuits, etc., as is wellknown in the relevant arts.

Filter circuits are often implemented using components such astrans-conductors and operational amplifiers. In some environments, itmay be desirable to implement filter circuits using trans-conductors atleast in that trans-conductors generally consume less electrical powerthan operational amplifiers. For further information ontrans-conductors, the reader is referred to a book entitled, AAnalogueIC Design: The current-mode approach@, by C. Toumazou, F. J. Lidge & D.G. Haigh, ISBN No.:086341 215 7, and is incorporated in its entiretyherewith.

However, one typical problem with trans-conductor circuits based filtercircuits is that the noise components introduced into the output signalsis generally high at least compared to operational amplifiers basedcircuits. One way to reduce the noise component is by operating thefilter circuits at high voltage levels or high power levels. However,such operation generally consumes more electrical power, and may beundesirable at least in some environments. In addition or in thealternative, the area (on an integrated circuit) of the trans-conductorcircuits may have to be increased to reduce the noise factor, which mayalso be undesirable in many environments.

What is therefore required is a filter circuit based on trans-conductorcircuits which satisfies one or more of the above-noted requirements.

SUMMARY OF THE INVENTION

A basic block implemented according to an aspect of the presentinvention may be used to implement a filter circuit. In an embodiment,the basic block contains a biasing circuit providing a biasing signal toset an operating point of a trans-conductor circuit and a common-modefeedback circuit providing a feedback signal to stabilize thetrans-conductor circuit, with the biasing signal and the feedback signalbeing combined and provided on a common path to the trans-conductorcircuit.

Due to the use of the common path, the number of components (includingtransistors) to implement the basic block may be reduced. As a result,the noise introduced by the components of a filter circuit and the totalelectrical power consumed may be minimized.

An embodiment of the basic block may further include a control circuitgenerating a quiescent voltage of the trans-conductor circuit, with thecontrol circuit being implemented to have a same (or similar) transferfunction as the trans-conductor circuit. The biasing circuit may set theoperating point of the trans-conductor circuit based on the quiescentvoltage also. In one implementation, the control circuit is implementedsimilar (in terms of components and connectivity) to the trans-conductorcircuit and operated on by using a D.C. voltage to generate thequiescent voltage of the trans-conductor circuit.

The common-mode feedback circuit may contain a common mode sense circuitgenerating a common mode voltage, and an error amplifier may amplify thecommon mode voltage and providing a resulting amplified output to thebiasing circuit. In an embodiment, the trans-conductor circuit isimplemented using PMOS transistors and the biasing circuit isimplemented using NMOS transistors.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.In the drawings, like reference numbers generally indicate identical,functionally similar, and/or structurally similar elements. The drawingin which an element first appears is indicated by the leftmost digit(s)in the corresponding reference number.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described with reference to theaccompanying drawings, wherein:

FIG. (FIG.) 1 is a circuit diagram illustrating the details of anexample device in which the present invention may be implemented;

FIG. 2 is a circuit diagram illustrating the details of an embodiment ofa filter circuit;

FIG. 3 is a circuit diagram illustrating the details of an embodiment ofa basic filter block implemented in accordance with the presentinvention; and

FIG. 4 is a circuit diagram of a basic filter block illustratingadditional implementation in an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Overview and Discussion of the Invention

According to an aspect of the present invention, a biasing signaldesigned to set an operating point of a trans-conductor circuit and acommon mode feedback signal for the trans-conductor circuit are combinedand provided on a common path to the trans-conductor circuit. As aresult, the number of components used to implement a filter circuit maybe reduced, leading to lower noise and reduction of electrical powerconsumption.

Several aspects of the invention are described below with reference toexample devices for illustration. It should be understood that numerousspecific details, relationships, and methods are set forth to provide afull understanding of the invention. One skilled in the relevant art,however, will readily recognize that the invention can be practicedwithout one or more of the specific details, or with other methods, etc.In other instances, well-known structures or operations are not shown indetail to avoid obscuring the invention.

2. Example Device

FIG. 1 is a block diagram of receiver device 100 illustrating an exampledevice in which the present invention may be implemented. Forillustration, it is assumed that receiver device 100 is implementedwithin a Global Positioning System Receiver. However, receiver device170 can be implemented in other devices (e.g., mobile phone, etc., inwhich reduction of power consumption is of importance) as well. Receiverdevice 100 is shown containing antenna 101, filter 110, low noiseamplifiers (LNA) 120 and 140, band pass filter 130, mixer 150, automaticgain controller 160, filter circuit 170, amplifier 180, analog todigital converter (ADC) 190, and processing unit 195. Each component isdescribed in further detail below.

Antenna 101 may receive various signals transmitted from satellites,etc. The received signals may be provided to filter 110. Filter 110 mayperform a corresponding transfer function to generate signals of thefrequencies of interest. The generated signals are provided to LNA 120.Antenna 101 and filter 110 may be implemented in a known way.

LNA 120 amplifies the signals received on line 112 to generate acorresponding amplified signal on line 123. Band pass filter (BPF) 130may filter the amplified signal to remove any unwanted noise componentsthat may be present. The filtered signal thus generated may be providedto LNA 140. LNA 140 may again amplify the filtered signals and providethe amplified filtered signal to mixer 150. LNAs 120 and 140, and BPF130 may also be implemented in a known way.

Mixer 150 may be used to convert a high frequency signal to a signalhaving any desired frequency. In an embodiment, a signal of frequency1575 MHz is converted to a 4 MHz signal. Mixer 150 may receive filteredamplified signal and a signal of fixed frequency as inputs. The signal(on path 151) of fixed frequency may be generated by a phase locked loop(not shown) in a known way.

Automatic gain control (AGC) 160 may be used to amplify or attenuate thesignal (from mixer 150) according to various requirements. For example,if a user using a mobile phone is in an area where the signals receivedare of low strength, and AGC 160 amplifies the signal accordingly.Similarly, if the user moves to an area where the signal strength isrelatively higher, AGC 160 may attenuate the signal.

Filter circuit 170 may remove any unwanted noise components present inthe signal received on line 167 to generate a filtered signal. Thefiltered signal may be provided to amplifier 180. Amplifier 180 mayfurther amplify the signal received on line 178 to generate an amplifiedsignal. The amplified signal may be provided to analog to digitalconverter (ADC) 190. It may be noted that all the above components ofFIG. 1 operate on signals that are analog in nature.

ADC 190 converts the analog signal received on line 189 to acorresponding digital signal. The digital signal on line 192 may then beprovided to processing unit 195 for further processing.

It may be desirable to minimise electrical power consumption or toreduce the degree of unwanted noise introduced in many components suchas low pass filter 170 and amplifier 180. The manner in which suchadvantages may be obtained is described below with reference to filtercircuit 170 for illustration.

3. Implementation of Filter Circuit

FIG. 2 is a block diagram illustrating the details of an embodiment offilter circuit 170. For illustration, filter circuit 170 is shown as asecond order filter in FIG. 2. However, filter circuit 170 may also beused to implement filters corresponding to any order. Filter circuit 170is shown containing basic filter blocks 251 through 254, capacitors 261through 262 and inverter 270. Each component is described in furtherdetail below.

Basic filter block (BFB) 251 receives an input signal with a voltagelevel of “Vin” on line 201 to generate a corresponding output signalwith a current level of “Iout” on line 211. The signals on line 211 maybe represented as follows:

Iout=Vin*Gm  Equation (1)

Vout=Iout*Impedance of capacitor 261  Equation (2)

wherein ‘Vin’ represents the voltage of the input signal, ‘Iout’represents the current of the output signal, ‘Gm’ represents thetrans-conductance of a trans-conductor circuit contained within BFB 251,and ‘*’ represents the multiplication operation, and Vout represents thevoltage on line 211.

Basic filter blocks 252-254 also operate similar to equations (1) and(2). Basic filter blocks 251-254 may be implemented using differentialtrans-conductor circuits. However, as noted above in the backgroundsection, trans-conductor circuits may introduce noise into the outputsignals and/or operate at high voltage. The manner in which basic filterblocks 251-254 may be implemented using differential trans-conductorcircuits while addressing such concerns is described below in furtherdetail.

4. Implementation of Basic Filter Block

FIG. 3 is a block diagram illustrating the details of an embodiment ofbasic filter block 251 implemented using trans-conductor circuit. Basicfilter block 251 is shown containing differential trans-conductorcircuit 310, biasing circuit 320, common mode feedback circuit 330, biasinterface circuit 340 and control circuit 350. Each component isdescribed below in further detail.

Differential trans-conductor circuit 310 may receive input signal (211)with a voltage level of Vin and generate a corresponding output signalsaccording to equations (1) and (2). In an embodiment, differentialtrans-conductor circuit 310 is implemented using PMOS transistors. Theoutput signal is shown provided to common mode feedback circuit 330.

Control circuit 350 may have the same transfer function (in terms ofconverting voltage to electric current) as trans-conductor circuit 310.However, control circuit 350 may operate using a direct current (dc)voltage such that the output of control circuit accurately represents aquiescent voltage (may approximately equal Vdd/2 on line 351). In anembodiment, control circuit 350 contains the same components andtopology as trans-conductor circuit 310, and may be implemented in aknown way.

Common mode feedback circuit 330 is shown containing common mode sensecircuit 335 and error amplifier 337. Common mode sense circuit 335 maybe designed to generate a common mode voltage on path 332. In addition,common mode sense circuit 335 may be designed to operate across a largefrequency range. Error amplifier 337 may amplify the signal generated bycommon mode sense circuit 335 to provide a feedback signal on path 332.

In an embodiment described below with reference to FIG. 4, erroramplifier 337 may be implemented within biasing circuit 320. Biasinterface circuit 352 may provide an interface between control circuit350 and biasing circuit 320.

Biasing circuit 320 generates a biasing signal on path 312. The biasingsignal biases (by providing a biasing signal) differentialtrans-conductor circuit 310 and sets the operating point. As is wellknown, the operating point generally refers to a steady-state operationof a circuit. In an embodiment, biasing circuit 320 may be implementedusing NMOS transistors as shown.

The biasing signal (generated by biasing circuit 320) and the feedbacksignal (generated on path 332) are combined and provided on path 312.Due to such combining, the number of components to implement the basicblocks may be minimized, leading to several advantages (such asreduction in noise introduced and power consumption). The description iscontinued with reference to the circuit level implementation of basicfilter block 251.

5. Circuit Implementation

FIG. 4 is a circuit diagram illustrating the details of an embodiment ofbasic filter block 251. FIG. 4 is shown containing PMOS transistors 411and 412, NMOS transistors 421 through 425, resistors 431 and 432,capacitors 441 and 442, and load 450. Each component is described infurther detail below. The relationship of the components of FIG. 4 withthe components of FIG. 3 is also noted for the convenience of thereader.

Gate terminal 401 of PMOS transistors 411 is shown connected to thepositive input signal (INP) and gate terminal 402 of PMOS transistor 412is shown connected to negative input signal (INM). The signals INP andINM together represent the differential input signal with a voltagelevel of Vin. The two transistors together generate a correspondingoutput signal Vout across load 450. The source terminal of both PMOStransistors 411 and 412 is connected to voltage Avdd (Supply) providedat point 405.

PMOS transistors 411 and 412 together form an embodiment of differentialtrans-conductor circuit 310 of FIG. 3. However, it will be apparent toone skilled in the arts to implement various alternative embodimentswithout departing from the scope and spirit of the present invention.For example, basic filter block 251 may be implemented using PMOStransistors in lieu of NMOS transistors and vice versa.

Resistors 431 and 432, capacitors 441 and 442, and NMOS transistors 423and 424 may together represent common mode sense circuit 335, and thesensed signal is provided back to NMOS transistors 421 and 422, whichprovided the function of biasing circuit 320. NMOS transistors 423 and424 may be implemented as source followers and used to provide commonmode feedback to NMOS transistors 421 and 422. Resistors 431 and 432,and capacitors 441 and 442 may be designed such that the common modevoltage Vcm may be obtained at point 460. Common mode voltage may berepresented as follows:

Vcm=(Vop+Vom)/2  Equation (3)

wherein ‘Vcm’ represents the common mode voltage, ‘Vop’ represents thepositive output voltage, ‘Vom’ represents the negative output voltage,‘+’ represents the addition operation, and ‘/’ represents the divisionoperation.

The biasing signal is received at the gate of NMOS transistor 425, whichis provided to the gate of NMOS transistors 421 and 422 to set theoperating point. NMOS transistor 425 may correspond to the interfacebetween control circuit 350 (shown only in FIG. 3) and NMOS transistors421 and 422. The drain terminal 460 of NMOS transistors 425 may beconnected to the gate terminals of NMOS transistors 421 and 422.

From the above, it may be appreciated that the common mode feedbacksignal is generated at node 460. Again 460 is connected to the drain ofNMOS transistor 425. Biasing circuit (via NMOS transistor 425) drivesnode 460 (gate terminal of NMOS transistors 421 and 422) to a desiredvoltage (as determined by the biasing circuit. Thus, the output commonmode voltage equals the voltage at node 460.

The pair of NMOS transistors 421 and 422 also acts as an error amplifierto ensure that the common mode voltage generated by R-C circuit is thedesired voltage determined by the biasing circuit. That is, both thesignals are getting combined at node 460 and applied to the basictrans-conductor block 411 and 412 by NMOS transistors 421 and 422.

By combining the common-mode feedback signal and the biasing voltage(which sets the operating point), and providing the combined signal onthe same path, the number of components in an integrated circuit may bereduced. The reduction in components generally implies reduced noise andfewer parasitic poles (implying ability to operate at high bandwidth).The reduction also leads to reduced power consumption.

Thus, a basic block provided in accordance with the present inventioncan be used in components such as filters and amplifiers to implementdevices such as cell phones, which may need to operate at high bandwidthwhile consuming minimal power.

6. Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent invention should not be limited by any of the above describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

What is claimed is:
 1. A basic block used to implement a filter circuit,said basic block comprising: a trans-conductor circuit generating anoutput in response to receiving an input; biasing circuit providing abiasing signal to set an operating point of said trans-conductorcircuit; a common-mode feedback circuit generating a feedback signalbased on said output and providing said feedback signal to saidtrans-conductor circuit, wherein said biasing signal and said feedbacksignal are combined and provided on a common path to saidtrans-conductor circuit; and a control circuit generating a quiescentvoltage of said trans-conductor circuit, wherein said control circuit isimplemented to have a same transfer function as said trans-conductorcircuit, said quiescent voltage being provided to said biasing circuit.2. The basic block of claim 1, wherein said control circuit isimplemented similar to said trans-conductor circuit and operated on byusing a D.C. voltage to determine said quiescent voltage.
 3. The basicblock of claim 1, wherein said common-mode feedback circuit comprises: acommon mode sense circuit generating a common mode voltage according tosaid input and said output; and an error amplifier amplifying saidcommon mode voltage and providing a resulting amplified output to saidbiasing circuit.
 4. The basic block of claim 3, wherein said erroramplifier is integrated into said biasing circuit.
 5. A devicecomprising: a basic block comprising: a trans-conductor circuitgenerating an output in response to receiving an input; biasing circuitproviding a biasing signal to set an operating point of saidtrans-conductor circuit; a common-mode feedback circuit generating afeedback signal based on said output and providing said feedback signalto said trans-conductor circuit; and a control circuit generating aquiescent voltage of said trans-conductor circuit, wherein said controlcircuit is implemented to have a same transfer function as saidtrans-conductor circuit, said quiescent voltage being provided to saidbiasing circuit, wherein said biasing signal and said feedback signalare combined and provided on a common path to said trans-conductorcircuit.
 6. The device of claim 5, wherein said control circuit isimplemented similar to said trans-conductor circuit and operated on byusing a D.C. voltage to determine said quiescent voltage.
 7. The deviceof claim 5, wherein said common-mode feedback circuit comprises: acommon mode sense circuit generating a common mode voltage according tosaid input and said output; and an error amplifier amplifying saidcommon mode voltage and providing a resulting amplified output to saidbiasing circuit.
 8. The device of claim 7, wherein said error amplifieris integrated into said biasing circuit.
 9. The device of claim 5,further comprising: an antenna receiving an external signal, whereinsaid input is generated based on said external signal; a filter circuitcomprising said basic block, and generating said output; an analog todigital converter converting said output to a plurality of digitalsamples; and a processing unit processing said plurality of digitalsamples.
 10. The invention of claim 9, wherein said device comprises oneof a mobile phone and a global positioning system receiver.
 11. A filtercircuit comprising: a trans-conductor circuit means generating an outputin response to receiving an input; biasing means providing a biasingsignal to set an operating point of said trans-conductor circuit;feedback means generating a feedback signal based on said output andproviding said feedback signal to said trans-conductor circuit means;and a control means generating a quiescent voltage of saidtrans-conductor circuit means, wherein said control means is implementedto have a same transfer function as said trans-conductor circuit means,said quiescent voltage being provided to said biasing means, whereinsaid biasing signal and said feedback signal are combined and providedon a common path to said trans-conductor circuit.
 12. The filter circuitof claim 11, wherein said control means is implemented similar to saidtrans-conductor circuit means and operated on by using a D.C. voltage todetermine said quiescent voltage.
 13. The filter circuit of claim 12,wherein said feedback means comprises: a common mode sense meansgenerating a common mode voltage according to said input and saidoutput; and amplifying means amplifying said common mode voltage andproviding a resulting amplified output to said biasing means.
 14. Thefilter circuit of claim 13, wherein said amplifying means is integratedinto said biasing means.
 15. The filter circuit of claim 11, whereinsaid trans-conductor circuit means comprises a plurality of PMOStransistors and said biasing means comprises a plurality of NMOStransistors.